The large increase in the use of narcotic agents, including the opium alkaloids, by the general population has brought with it a substantial need to improve analytical techniques for the determination of such agents in biological fluids. In many instances, medical treatment centers are faced with the need for determining the identity of a narcotic agent taken by a patient who, being in a comatose condition, is unable to supply such information to the treating physician. Early procedures involved the identification of opium alkaloids by extraction and thin-layer chromatographic methods. These techniques have the disadvantages of being relatively time-consuming, laborious and lacking great sensitivity. Recently, a rapid and sensitive immunoassay procedure involving the reaction between antibodies and opium alkaloid antigens was described in U.S. Pat. No. 3,709,868. However, this procedure requires sophisticated equipment such as scintillation counters to be used for the assay. It would, therefore, be desirable to develop a rapid and highly sensitive assay for detecting the presence of opium alkaloids in biological fluids which would not require sophisticated equipment and could be easily performed by laboratory technicians having a minimum of training.